1. Field of the Invention
The present invention relates to a method for filling material beads, a method for die-expanded molded foaming of synthetic resin using the above method, and a die-expanded molded foam product obtained thereby, in a die-expanded molded foaming apparatus which molds molded foam products using material beads made of thermoplastic synthetic resin, such as polyolefine and polystyrene.
2. Description of the Related Art
As a die-expanded molded foaming apparatus for manufacturing molded foam products using material beads made of thermoplastic synthetic resin, an apparatus shown in FIG. 11 has been commercialized, wherein a set of dies 100 and 101 are disposed opposite each other, chambers 102 and 103 are disposed at the rear side of the dies 100 and 101 respectively, many vent holes 105 and 106 connecting the chambers 102 and 103 and a cavity 104 are formed in both of the dies 100 and 101, and utility fluid, such as the later mentioned heating steam, is supplied to the cavity 104 or is exhausted from the cavity 104 through the vent holes 105 and 106. In the structure of this case, upper utility ports 107 and 108 for supplying heating steam are disposed at the upper part of the respective chambers 102 and 103, and lower utility ports 109 and 110 connected to a pressure reducing pump or a drain pipe are disposed at the lower part of the respective chambers 102 and 103 so that steam is supplied to the cavity 104.
For actually forming the many vent holes 105 and 106 opened in the dies 100 and 101, a core vent 111, which is a cylindrical body with a cap having an outer diameter of 7xcx9c12 mm where a plurality of vent holes 105 and 106, which are approximately 0.5 mmxcfx86 round holes or approximately 0.5 mm width slits, are opened, is embedded in the respective core vent attachment holes 112, which are opened at a 20xcx9c50 mm pitch in the dies 100 and 101, as illustrated in FIG. 12 and FIG. 13.
In order to mold a molded foam body using such a molded foaming apparatus, preexpanded material beads made of thermoplastic synthetic resin, such as polyolefine and polystyrene, are supplied from a material tank (not illustrated) and filled into the cavity 1 via a filling unit 113, are heated with the heating steam for foaming and fusing, are cooled and solidified, and then are taken out as a molded foam body.
Widely used methods for filling the material beads are 1) a cracking method, 2) a pressure filling method, and 3) a compression filing method, and these methods will be briefly described first.
1) The cracking method is a method for filling material beads by mechanically supplying the mechanical beads using an injector in a state where the material tank and the cavity are opened to atmospheric pressure, and has a disadvantage that air remaining inside is apt to cause filling unevenness, and increase the dispersion of filling density.
2) The pressure filling method is a method for filling the material beads by pressurizing inside the material tank to 0.2xcx9c1.5 kg/cm2, and supplying and filling the material beads into the cavity, which is opened to atmospheric pressure, through the chamber using the differential pressure. In this case, filling unevenness is less than the above mentioned cracking method, since differential pressure in the carrier passage from the material tank to the cavity can be utilized.
3) The compression filling method is a method for supplying and filling material beads by pressurizing the pressure P inside the material tank to 1.0xcx9c5.0 kg/cm2, slightly higher than the pressure filling method, and pressurizing one chamber so as to change the differential pressure (Pxe2x88x92P1) with the pressure P1 inside the cavity connected thereto via vent holes, and the filling property is good since the material beads are supplied in a compressed state.
Now, a method for heating, expanding and fusing the material beads, which are filled in the cavity as mentioned above, will be examined. As a heating expansion method for the material beads, a method stated in the Japanese Patent Laid-Open No. 57-174223 has been generally used. In this publication, the process drawing shown in FIG. 14 is included, wherein (a)xcx9c(d) depict the preheat exhausting process for substituting air in the dies and air among the material beads with steam, and the concrete content of each process will be explained below. In FIG. 14, a black valve symbol indicates that the valve is closed, and a white valve symbol indicates that the valve is open.
(a) is an exhaust process, wherein after the material beads are filled in the cavity 104, steam is supplied from the top utility ports 107 and 108 to the chambers 102 and 103 for a very short time, and at the same time, air in the dies, particularly in the chambers 102 and 103, is exhausted from the bottom utility ports 109 and 110 by suction. In this case, pressure inside the chambers 102 and 103 is increased to be a plus pressure by steam so that steam enters among the material beads through the vent holes 105 and 106.
(b) is a both-side exhaust process, wherein the top utility ports 107 and 108 are closed, and the vacuuming and pressure reducing operation is continued to reduce pressure inside the dies so that air existing in spaces among the material beads is sucked and exhausted through the vent holes 105 and 106 formed in the dies on both sides.
(c) is a one-side preheating process, wherein the bottom utility ports 109 and 110 are closed and steam is supplied from the top utility port 108 of one chamber 103, which is in a pressure-reduced state, for a short time. In this case, the supplied steam passes sequentially through the vent hole 106 of the die 101, among the material beads in the cavity 104, and the vent hole 105 of the die 100, and flows into the chamber 102 at the opposite side, by which all of the material beads and the dies 100 and 101 are preheated.
(d) is a one-side preheating process with an opposite flow of steam, wherein a similar operation is carried out from the chamber 102 side so that air in the cavity 104 is completely exhausted, and at the same time, both dies 100 and 101 are preheated while decreasing the localized temperature difference as much as possible.
(e) is a heating process for fusion, wherein steam for heating for fusion is supplied to both of the chambers 102 and 103 for heating the dies 100 and 101, and for heating the materials beads as well through the vent holes 105 and 106 of the respective dies 100 and 101, so as to complete foaming, and to mutually fuse the material beads to form the molded foam body.
In the case of the above mentioned molding method, wherein such a utility fluid as air and steam is supplied to the cavity or is exhausted from the cavity, the vent holes are indispensable, but forming vent holes will cause the following problems.
(1) In order to compensate for strength which is decreased by opening many core vent attachment holes in the dies, the wall thickness of a die made of aluminum alloy material must be set thick, 8xcx9c12 mm for example, which increases heat capacity, causing such problems as poor heat efficiency for heating and cooling, or a slow speed of temperature rising and lowering, which drops control accuracy.
(2) Since 2000xcx9c4000 core vent attachment holes are opened in a general pair of dies, a complicated drilling operation increases processing cost, and since the core vents are attached manually, this operation is complicated, causing inevitable damage on the surface of the dies, where an extra operation for repair is necessary.
(3) Vent holes are clogged by e.g. scales, causing heat failure, mold releasing failure, and cooling failure, which makes a maintenance operation necessary, such as replacing core vents or periodic cleaning by high pressure cleaning water.
(4) Traces of core vents and vent holes remain on the surface of the molded foam product, which causes a drop in the beauty of the appearance of the molded foam product, and when the outer surface is printed, traces of core vents and vent holes become the cause of dropping printing quality.
(5) After molding, the molded foam product is cooled down by spraying cooling water into the chambers, and at this time moisture infiltrates into the cavity through the vent holes, resulting in the molded foam product containing 6xcx9c10% water inside, which makes a drying process necessary. Also the cooling water must be controlled to be a clean state to obtain a clean molded foam product, since the cooling water directly contacts the molded foam product.
(6) Since the material beads are heated for expanding and fusing under the same heating conditions by supplying steam from the chambers to the cavity, the surface property of the molded foam product obtained like this (hereafter equal heating molded foam product) changes depending on the fusion rate of the beads. Concretely, the surface property worsens as the fusion rate decreases, and surface property improves as the fusion rate increases. On the other hand, the higher the fusion rate of the beads is set for an equal heating molded foam product, the better properties become, such as the mechanical strength of the molded foam product, but heating, expanding and fusing time, and cooling time become longer, which makes the general cycle time of molding longer, decreasing productivity.
For the above reasons, in the above mentioned molding technology, the fusion rate of beads of a molded foam product is set to e.g. 40xcx9c80% so as to improve surface property to insure beauty in appearance and to insure mechanical strength by setting a sufficiently high fusion rate, however the fusion rate must be set high enough to insure beauty in appearance even for a molded foam product which does not demand high mechanical strength, therefore cycle time for molding increases and productivity decreases. The fusion rate here is based on an evaluation of the state of beads in a cross-section when the molded foam product is split, and more concretely, the ratio of broken beads determined by measurement, regarding a bead as not fused when the bead itself is not broken but has a crack along its surface, and regarding a bead as fused when the bead itself is broken.
As described above, in conventional molded foaming methods, where such a utility fluid as steam and air is supplied to the cavity or is exhausted from the cavity using the vent holes to obtain a molded foam product, forming the vent holes cause the many problems mentioned above.
To completely solve these problems, the present inventors examined practical methods for molded foaming using dies without the vent holes, and conducted various tests. Even if the vent holes are not formed in the dies, passages which substitute for the vent holes are indispensable for supplying or exhausting such utility fluid as steam and air to/from the cavity, so there are many problems including where and how such passages are to be formed and at which timing and under what conditions the utility fluid is to be supplied to these passages.
One of such problems is a problem concerning a method for filling the material beads to the cavity.
When the above mentioned cracking method is used, material beads can be supplied up to a point by leaving some space open in the set of the dies without being completely closed, but filling is so uneven that it is impossible to apply this method for practical use.
In the case of the pressure filling method, which can maintain the inside of the cavity to atmospheric pressure by leaving some space open in the set of the dies, filling operation is possible up to a point, but the filled material beads are compressed for the closing dimensions corresponding to cracking after the filling operation, so when a box shaped molded foam product which top face is open is manufactured, for example, compressibility differs between the plane portion to be the bottom of the molded foam product and the portion to be the side wall, where the filling factor is greater in the plane portion than in the side wall portion, still causing filling density unevenness.
In the case of the compression filling method, the inside of the cavity must be maintained to be a predetermined pressure, which is atmospheric pressure or higher, however we cannot compare and examine this method since there is no means for supplying the material beads with dies which have no vent holes while maintaining pressurization.
It is an object of the present invention to commercialize a new molded foaming apparatus where the vent holes are not formed in the die faces, and to provide a method for filling the material beads which can minimize filling unevenness during filling the material beads, which is one of the problems of the molded foaming apparatus having no vent holes, a method for die-expanded molded foaming of synthetic resin using the filling method, and to provide a die-expanded molded foam product obtained thereby.
A method for filling material beads according to a first aspect of the present invention is a method for filling the material beads from a material tank to a cavity comprised of a set of dies, wherein the cavity is formed enclosed with a pair of dies with chambers disposed on the rear side respectively, and is air tightly isolated from the chambers, then the material beads are supplied to the cavity while maintaining the pressure in the cavity at a predetermined minus pressure with respect to the pressure in the material tank.
In the case of the filling method according to the first aspect, the chambers and the cavity are air tightly isolated, that is, the molded foaming apparatus to be used does not have vent holes for connecting the chambers and the cavity in the dies, therefore the above mentioned problems caused when forming vent holes can be fundamentally solved. Filling the material beads, which utilizes differential pressure between the material tank and the cavity, can be easily carried out by adjusting the pressure of the material tank and the cavity.
The filling method according to the second aspect is that the material beads are supplied to the cavity while maintaining the pressure in the above mentioned material tank in an atmospheric pressure xcx9c5.0 kg/cm2 range. This configuration is preferable since filling density becomes relatively uniform in the plane portions, the side face portions or even in the narrow portions of the cavity, by compressing and decreasing the capacity of the material beads in the cavity to be an optimum outer diameter (capacity).
A filling method according to a third aspect is that the pressure in the cavity is maintained to be a pressurized state which is less than minus 0.5 kg/cm2 with respect to the pressure in the material tank and the atmospheric temperature or more. It is preferable to set to this range because in a pressure state outside this range, the flow velocity of the carrier air flow becomes excessive, and clogging of the material beads occurs in the cavity or in a narrow portion of the filling passage.
A filling method according to a fourth aspect is that the pressure in the cavity is maintained to be a reduced pressure state which is from atmospheric pressure to a minus 1.0 kg/cm2 range. In this case, the pressure in only the cavity is reduced for the material beads which do not need compression and decreasing capacity, which is preferable because a negative pressure state is easily maintained during filling, and filling density becomes uniform in the plane portions or the side face portions or even in the narrow portions of the cavity.
After considering claims 1 to 4 in general, it is preferable to set the pressure in the cavity in a minus 1.0xcx9c5.0 kg/cm2 range, which is a fifth aspect.
A method for die-expanded molded foaming of synthetic resin according to a sixth aspect is a method for die-expanded molded foaming of synthetic resin comprising the steps of: filling material beads in a cavity for molded foaming enclosed with a pair of dies with chambers disposed on the rear side respectively; heating, expanding and fusing the material beads to be a molded foam body; and cooling, releasing and taking out the molded foam body from the dies, wherein the following processes are included.
a) A die closing process where the dies are closed to form the cavity air tightly isolated from the chambers and to form utility passages directly connected to the cavity.
b) A filling process where the material beads are supplied and filled into the cavity from the material tank by the method for filling material beads according to one of the first to fifth aspects, and at the same time heating steam is supplied to the chambers so as to heat the entire dies around the chambers to a temperature where the material beads do not expand.
c) An internal heating process where heating steam having a temperature less than the fusing temperature of the material beads is supplied to the cavity through the utility passages for the cavity so as to heat the filled material beads, and at the same time, condensed water is generated on the faces of the dies enclosing the cavity.
d) A fusion heating process where heating steam having a temperature at the fusion temperature of the material beads or higher is supplied to the chamber so as to evaporate the condensed water in the cavity and to progress the expansion and fusion of the material beads.
According to this molded foaming method, just like the filling method in accordance with the first aspect, the chambers and the cavity are air tightly isolated, that is, the molded foaming apparatus to be used does not have vent holes for connecting the chambers and the cavity in the dies, and steam and air are supplied to or exhausted from the cavity, not through vent holes but through utility passages directly connected to the cavity, therefore the above mentioned problems caused when forming the vent holes can be fundamentally solved by the setting of the utility passages.
Since the material beads are filled by a filling method according to one of the first to fifth aspects in the filling process, filling can easily be uniform by adjusting the pressure of the material tank and the cavity.
Also the utility fluid can be supplied to the cavity and the chambers independently, therefore preheating of the dies can be operated by supplying heating steam to the chambers separately from the filling operation. In other words, it is preferable to preheat the dies, entered from the closing process to the filling process, preparing for the expansion operation in the after process, even when molding is continuously performed because time has passed since the last heating operation and temperature of the dies has considerably dropped, but in a conventional filling process, where air for filling the material beads is exhausted from the cavity via the chambers, the dies cannot be preheated in this filling process stage. Whereas in the present invention, where the exhaust passage for air for filling is independent from the chambers, preheating for the dies can be operated in parallel by supplying heating steam to the chambers separately from the filling operation.
A method for die-expanded molded foaming according to a seventh aspect is that in the die closing process, a die-expanded molded foaming apparatus is used, where slit shaped openings opening to the cavity and/or openings opening to the cavity near an accessory part to be attached at least to one of the dies are formed along the joint of the dies to be the opening width which is an outer diameter size or less of a material bead to be filled, and at the same time, connecting passages connecting the openings to external utility pipes are disposed.
By using such a die-expanded molded foaming apparatus, core vent attachment holes can be completely eliminated, manufacturing cost can be decreased by constituting the dies thin, and control accuracy of such utility fluid as steam can be controlled. Since operation for forming the core vent attachment holes and operation for attaching the core vents are decreased or eliminated, productivity of the dies can be improved, and maintenance operation required for clogging of the vent holes by scales can be decreased or eliminated. The openings formed at such positions do not drop beauty in appearance of the molded foam product, and such utility fluid as steam can be evenly supplied to the cavity.
The method for die-expanded molded foaming according to an eighth aspect is that the above mentioned accessory part is a filling unit of material beads. Since traces of the end face of the filling unit always remain on the surface of the molded foam product, the openings are formed near the filing unit so that traces of the openings are not outstanding, and a drop in appearance of the molded foam product is prevented.
The method for die-expanded molded foaming according to a ninth aspect is that a returning-to-atmosphere process for releasing the inside of the cavity from the pressurized state to an atmospheric pressure state or an exhaust process for reducing the pressure inside the cavity to exhaust internal air is added between the filling process and the internal heating process.
If the returning-to-atmosphere process is added like this, the material bead particles which have been pressurized and compressed expand as pressure is reduced and fill the mutual spaces, therefore the percentage of void decreases from 40% to 5xcx9c20%, decreasing the residual air content in voids among particles which will be a problem during fusion in the after process, and as a result uniformity inside the molded foam body improves. If the exhaust process is added, the pressure inside the cavity is reduced by actively exhausting air inside, and the percentage of void and residual air content can be decreased, and as a result uniformity inside the molded foam body improves, just like the returning-to-atmosphere process.
A method for die-expanded molded foaming according to a tenth aspect is that heating conditions for a pair of chambers and a cavity are controlled in a state where material beads made of thermoplastic synthetic resin are filled in the cavity by the filling method according to one of the first to fifth aspects, and the material beads filled in the cavity are heated, expanded and fused while controlling the internal fusion rate at will with maintaining the surface property of the molded foam product.
According to this molding method, wherein supply steam to the pair of chambers and cavity are supplied independently and the heating conditions for respective spaces are adjusted independently, the surface property of the material beads filled in the cavity contacting the dies can be adjusted respectively using the steam to be supplied to the pair of chambers, and the fusion rate of the material beads can be adjusted independently from the surface property by heating, expanding and fusing the material beads filled in the cavity using steam to be supplied to the cavity. This makes it possible to decrease the cycle time of molding while keeping the internal fusion rate of the molded foam product low, and to manufacture molded foam products with a beautiful surface as well, and consequently both productivity and commercial value can be implemented.
Since the chambers and cavity are formed in an air tightly isolated state, heating conditions for these three spaces can be more accurately set, and the above mentioned problems caused when forming the vent holes can be fundamentally solved. Vent holes connecting the cavity and the chambers may be formed only if the vent holes do not have a negative influence on control of the utility fluid. If the vent holes are formed in the core die and/or the cavity die, traces of the vent holes remain in the molded foam product, so it is preferable to form the vent holes such that the traces come to a location not outstanding in the molded foam product.
The method for die-expanded molded foaming according to an eleventh aspect is that the internal fusion rate is controlled at will with maintaining the surface property of the molded foam product, while independently controlling the heating conditions for the pair of chambers and the cavity. The pair of chambers may be controlled together, but it is preferable to control the respective chambers independently since the surface property of a surface at the core die side and the surface property of a surface at the cavity die side of the molded foam product can be controlled independently.
The method for die-expanded molded foaming according to a twelfth aspect is that the steam pressure and the time are controlled as heating conditions. It is also possible to control the steam temperature as a heating condition, but controlling the steam pressure and the time is preferable, since conventional steam equipment which has been used at a factory can be used, and equipment for control can be constructed at low cost.
The method for die-expanded molded foaming according to a thirteenth aspect is that the material beads are polyolefine synthetic resin material. Polyolefine synthetic resin material is preferable in terms of easy filling of the material beads even in a narrow portion, because the raw material itself is soft and has high gas permeability, which makes it much easier to change the shape of particles than material beads made of polystyrene synthetic resin material having the same expansion ratio.
A die-expanded molded foam product according to a fourteenth aspect is a molded foam product which is molded using material beads made of thermoplastic synthetic resin and has a beautiful surface free from traces of core vents and core vent holes. Such molded foam products with a beautiful surface free from traces of core vents and core vent holes are preferable since clean printing is possible on the outer surface. Such molded foam products can be easily manufactured by a molded foaming method according to one of the tenthxcx9cthirteenth aspects.
The die-expanded molded foam product according to a fifteenth aspect is that the internal fusion rate is lower than an equal heating molded foam product, having the same surface property setting, obtained by heating the surface and the inside under the same heating conditions. Such a molded foam product can decrease heating, expanding and fusion time during the molding and the cooling time, and can improve productivity by decreasing the cycle time for molding, since the internal fusion rate is lower than an equal heating molded foam product having the same surface property setting. Also both productivity and commercial value can be implemented since the surface property can be sufficiently maintained while setting the internal fusion rate low, without dropping commercial value.
The die-expanded molded foam product according to a sixteenth aspect is that the internal fusion rate is higher than an equal heating molded foam product, having the same surface property setting, obtained by heating the surface and the inside under the same heating conditions. Such a molded foam product has higher mechanical strength since the internal fusion rate is higher than an equal heating molded foam product having the same surface property setting, and is suitable when the surface property of the molded foam product is not a major requirement but only strength is demanded. In other words, such a molded foam product has higher mechanical strength since the internal fusion rate is higher than an equal heating molded foam product having the same surface property setting, and when this molded foam product is manufactured by a molding method according to the first aspect, heating steam pressure in the heating process can be set high only for the molding space formed by the cavity die and the core die, and therefore the amount of steam to be used is less than the case of molding an equal heating molded foam product, and energy can be saved.